We will examine the effect of digitalis toxicity on myocardial mechanics independent of changes produced by dysrhythmias. This will be done by examining mechanics in isolated right ventricular papillary muscles from cats in which dysrhythmias have been produced by daily administration of digitoxin. Effects on passive stiffness will be determined from elastic stiffness-stress relations as described by Mirsky (Prog. Cardiovs. Dis. 18:277, 1976). The effect on active mechanical properties will be determined by measuring force development and the maximal rate of force development at the peak of the length-resting tension curves. Additional studies will examine changes in right and left ventricular collagen, measured as hydroxyprolene, occurring during regression of right ventricular hypertrophy. In these studies cats will undergo pulmonary artery banding for various periods, following which the band will be removed and measurements of right ventricular mass and hydroxyproline made at selected intervals thereafter.